Martensite-austenite Constituent Research Articles

Effects of martensite-austenite constituents on mechanical properties of heat affected zone in high strength pipeline steels-review

To improve oil and gas pipelines reliability through the minimization of defects in the welded joint, many researches were conducted to control local brittle zones in weld joints, mainly Martensite-Austenite constituents (M-A). The latter may present toughness problems for the Heat Affected Zone (HAZ) in American Petroleum Institute (API) grade steels. Given the complexity of this problem due to several parameters involved in the behavior of M-A constituents and consequently the performance of the HAZ, the recent literature was reviewed. The formation, quantity and morphology of M-A in the HAZ of multi-pass welds were elucidated. The inherent potency of M-A constituents in deteriorating toughness and initiating fracture was highlighted. The purpose is to give a state of the art and extract the points not treated yet, related to the interaction between M-A constituents, metallurgical design, welding thermal cycle and toughness in HAZ of pipeline steels. In this article, a particular attention was given to the yet not clarified behavior of Niobium (Nb) and Titanium (Ti) carbonitride, in the case of two pass welding and their ability to control Austenite grain growth.

High-resolution characterization of the martensite-austenite constituent in a carbide-free bainitic steel

The multiphase microstructure of carbide-free bainitic steels comprises bainitic ferrite laths, retained austenite with different morphologies, a minor fraction of carbides and so-called martensite-austenite areas, which partially transform during the last cooling step. While the other constituent received much attention, little is known about the structure of the martensite-austenite constituent in carbide-free bainitic steels. Thus, in this study, it was structurally and chemically investigated by high-resolution techniques such as transmission electron microscopy and atom probe tomography after preceded unambiguous identification by electron backscatter diffraction in conventional as well as transmission mode. The results, ranging from carbon segregation to cementite precipitation in the martensitic part, indicate strong auto-tempering during final cooling which is followed by aging. Also, some kind of structural modulation in the austenite belonging to the martensite-austenite areas was observed. Atom probe tomography revealed a heterogeneous carbon distribution, further supporting the findings by transmission electron microscopy.

Effect of Ti content and martensite–austenite constituents on microstructure and mechanical property

ABSTRACTIn this study, the relationship between impact toughness and microstructure in Cr–Mo–V multi-pass weld metals has been systematically investigated. The Charpy impact energy of two weld metals with various alloy elements increased remarkably. The primary cause of the change of impact toughness was attributed to the difference of acicular ferrite (AF) content and prior-austenite grain size, and the size and distribution of necklace martensite–austenite (M–A) constituents. With increasing Ti content, Ti-containing inclusions were increased, which resulted in an increased number of nucleation sites for AF, a change in the microstructure from allotriomorphic ferrite to AF, and refined ferrite grain size. In addition, smaller and more dispersive M–A constituents were observed in the weld metal with higher impact toughness.

Impact Toughness of Subzones in the Intercritical Heat-Affected Zone of Low-Carbon Bainitic Steel.

The subzones of the intercritical heat-affected zone (IC HAZ) of low-carbon bainitic steel were simulated by using a Gleeble-3500 simulator to study the impact toughness. The results showed that the IC HAZ is not entirely brittle and can be further divided into three subzones according to the impact toughness or peak welding temperature; the invariant subzone heated between the critical transformation start temperature (Ac1) and 770 °C exhibited unchanged high impact toughness. Furthermore, an extremely low impact toughness was found in the embrittlement subzone, heated between 770 and 830 °C, and the reduction subzone heated between 830 °C and the critical transformation finish temperature (Ac3) exhibited toughness below that of the original metal. The size of the blocky martensite-austenite (M-A) constituents was found to have a remarkable level of influence on the impact toughness when heated below 830 °C. Additionally, it was found that, once the constituent size exceeds a critical value of 3.0 µm at a peak temperature of 770 °C, the IC HAZ becomes brittle regardless of lath or twinned martensite constitution in the M-A constituent. Essentially, embrittlement was observed to occur when the resolved length of initial cracks (in the direction of the overall fracture) formed as a result of the debonding of M-A constituents exceeding the critical Griffith size. Furthermore, when the heating temperature exceeded 830 °C, the M-A constituents formed a slender shape, and the impact toughness increased as the area fraction of the slender M-A constituents decreased.

Characterization of martensite-austenite constituents and micro-hardness in intercritical reheated and coarse-grained heat affected zones of API X70 HSLA steel

The thermal cycles that high strength low alloy (HSLA) low carbon steels experience during welding inevitably affect the microstructure and mechanical properties of the weld joints. It is generally agreed that the coarse-grained heat affected zone (CGHAZ) and the intercritical reheated coarse-grained heat affected zone (ICRCGHAZ) in one-pass and multi-pass welding, respectively, have the poorest microstructure and mechanical properties relative to the rest of the steel joint. The martensite-austenite (M-A) constituents formed in the HAZ regions predominantly govern property deterioration in the CGHAZ and the ICRCGHAZ by promoting the initiation and propagation of micro-cracks. However, the characteristics of the M-A constituents, i.e., fraction, size, shape and aspect ratio, are strongly affected by the welding heat input. Accordingly, in this study, the aim was to evaluate the influence of a cold wire addition in conventional tandem submerged arc welding (TSAW + cold-wire) on the microstructure and micro-hardness in the CGHAZ and ICRCGHAZ. Microstructural characterization showed that a lower fraction of finely distributed M-A constituents associated with smaller prior austenite grains (PAGs) formed in the CGHAZ of the steel welded by the TSAW + cold-wire process (CW-OD weld) relative to that welded by the conventional TSAW process (TW-OD weld). Furthermore, the fraction of M-A constituents in the ICRCGHAZ of the CW-OD weld was decreased. A higher fraction of slender shaped M-A constituents in the ICRCGHAZ of the TW-OD weld resulted in an increase in the micro-hardness. The microstructure and micro-hardness changes associated with cold wire addition were attributed to the lower welding heat input and, consequently, the faster cooling rate with a corresponding reduction in the retention time in the austenitic region and ferrite/austenite region in the CGHAZ and ICRCGHAZ regions, respectively.

Effect of Different Thermomechanical Processes on the Microstructure, Texture, and Mechanical Properties of API 5L X70 Steel

A commercial API 5L X70 steel plate was subjected to different thermomechanical processes to propose a novel thermomechanical rolling path to achieve improved mechanical properties. Scanning electron microscopy, electron backscatter diffraction, and x-ray texture analysis were employed for microstructural characterization. The results showed that strain-free recrystallized {001} ferrite grains that developed at higher rolling temperature could not meet the American Petroleum Institute (API) requirements. Also, refined and work-hardened grains that have formed in the intercritical region with high stored energy do not provide suitable tensile properties. However, fine martensite–austenite constituents dispersed in ferrite matrix with grains having predominantly {111} and {110} orientations parallel to the normal direction that developed under isothermal rolling at 850 °C provided an outstanding combination of tensile strength and ductility.

Investigation of local brittle zone in multipass welded joint of NiCrMoV steel with heavy section

Abstract

Experimental–numerical study on strain and stress partitioning in bainitic steels with martensite–austenite constituents

To achieve safety and reliability in pipelines installed in seismic and permafrost regions, it is necessary to use linepipe materials with high strength and ductility. The introduction of dual-phase steels, e.g., with a bainite and dispersed martensite–austenite (MA) constituent, would provide the necessary ingredients for the improvement of the strain capacity (as required by a new strain-based linepipe design approach) and toughness. To fine-tune the alloy design and ensure these dual-phase steels have the required mechanical properties, an understanding of the governing deformation micromechanisms is essential. For this purpose, a recently developed joint numerical–experimental approach that involves the integrated use of microscopic digital image correlation analysis, electron backscatter diffraction, and multiphysics crystal plasticity simulations with a spectral solver was employed in this study. The local strain and stress evolution and microstructure maps of representative microstructural patches were captured with a high spatial resolution using this approach. A comparison of these maps provides new insights into the deformation mechanism in dual-phase microstructures, especially regarding the influence of the bainite and MA grain size and the MA distribution on the strain localization behavior.

Structure and crystallography of martensite–austenite constituent in the intercritically reheated coarse-grained heat affected zone of a high strength pipeline steel

The structure and crystallography of martensite-austenite (M-A) constituent in the intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of X100 (690 MPa) pipeline steel weld joint was studied via multi-scale characterization. The results suggested that majority of the necklace-type M-A constituent in the ICCGHAZ preferred to form lamellar lath structure, which primarily consisted of lath martensite (87%). Only small fraction of retained austenite (9%) was found between martensite laths. The retained austenite had K-S orientation relationship with its neighboring martensite: (10-1)M // (11-1)γ and [111]M // [011]γ. Adjacent martensite laths within M-A constituent had large misorientation but no fixed crystallographic orientation relationship which implied that martensite laths may nucleate independently and encounter with each other during growth. M-A constituent and matrix microstructure belonged to different prior austenite grains. The martensite laths within the M-A constituent did not inherit the crystallographic orientation of the parent matrix during the intercritical reheating and subsequent cooling process of the second pass welding.

Microstructure and Properties of HAZ in Low-Carbon Bainite E550 Steel during Double-Pass Welding Thermal Cycle

Investigations on the microstructure and properties of the Coarse-Grained Heat-Affected Zone (CGHAZ) and intercritical reheated Coarse-Grained Heat-Affected Zone (ICCGHAZ) of a low-carbon bainite E550 steel were carried out using thermal simulation technology in this paper.Double-pass welding thermal cycle were performed on Gleeble-3800 thermal simulator, tempering heat treatment of the critical coarse crystal zone carried out in a box resistance furnace, low impact energies at -40 °C and Vickers hardness determined, and the microstructure were observed. The experimental results show that the microstructure of CGHAZ (Tp1 is 1320 °C) was dominated by coarse granular bainite and Lath bainite Ferrite, the impact toughness of CGHAZ was poor. The toughness of the CGHAZ was improved after second welding heat cycle except intercritical two-phase heating. When the peak temperature of the second thermal cycle(Tp2) was 650 °C, martensite-austenite (M-A) constituent of original CGHAZ wasdecomposed and refined, impact toughness and hardness were all higher than that of CGHAZ; When Tp2 is 750 °C, there was a ” necklace” distribution of massive M-A constituent in this ICCGHAZ, the impact energy at -40 °C prominently decreased and Hardness went up; When Tp2 was in the temperature range of 850 °C ~1100 °C, the microstructure was mainly finer granular bainite, the toughness of CGHAZ could be effectively improved; When Tp2 was over 1100 °C, M-A constituents become coarse, the toughness declined slightly . The changing of hardness was the opposite of toughness but the hardness fluctuation was comparatively small. After tempering at different temperature (520 °C~640 °C) , the grain boundary "necklace" structure of ICCGHAZ was still obvious, some of the M-A constituent were decomposed, the hardness decreased, the lowest hardness was obtained in 610 °C.

Thermal processes, microstructure, and mechanical properties near weld toe in double-sided double gas tungsten arc backing welding joint of 10CrNi3MoV steel

This study focused on heat-affected zone thermal processes, microstructural evolution, and impact toughness near weld toe of backing passes in double-sided double arc welding where there is stress concentration. The results show that as the arc distance increases, the second peak temperature near weld toe will accordingly decrease and the intercritically reheated coarse-grained heat-affected zone will appear. The formation of martensite-austenite constituents distributed both along the prior austenite grain boundaries and inside prior austenite grains leads to the poor toughness of intercritically reheated heat-affected zone. When short arc distance is adopted, supercritically reheated coarse-grained heat-affected zone will appear near weld toe which possesses high toughness due to the good toughness of lath bainite and the fine bainite/martensite packets. During double-sided double arc backing welding, short arc distance should be used to obtain excellent impact toughness.

Effects of martensite-austenite constituent on crack initiation and propagation in inter-critical heat-affected zone of high-strength low-alloy (HSLA) steel

Crack susceptibility of martensite-austenite constituent (MA) in inter-critical heat-affected zone (ICHAZ) is varied with properties of adjacent matrix, hardness of MA, and MA/matrix interfacial characteristics, but reasons why MAs are voided at MA/matrix interfaces or cracked at themselves still remain to be addressed. Effects of microstructural features including MA and matrix composed of granular bainite (GB) and quasi-polygonal ferrite (QPF) on crack initiation and propagation behavior, which affected critical crack tip opening displacement (CTOD), were investigated in this study. In the ICHAZ mainly composed of GB (75.2vol%) and QPF (19.5vol%), together with 5.3vol% of MA, QPF areas were more deformed than GB areas during the interrupted three-point bending tests, while some MAs were voided or cracked as MA areas were also highly deformed. The Nano-indentation test data indicated that the hardness of MA depended on boundary characteristics of nearby matrix, e.g., QPF having high-angle boundaries vs. GB having low-angle boundaries. According to detailed analyses on MA/QPF interface by using an atom probe, C, Mn, Mo, Ni, and Cr atoms were segregated at the MA/QPF interface, whereas Si, Cu, Al, and P were hardly segregated. These results indicated that major alloying elements (C, Mn, Ni, and Cr) were segregated at the MA/QPF interface, which influenced the deterioration of critical CTOD by reducing the mean free path for the crack initiation and propagation.

Prediction of steel weld HAZ Charpy impact property based on stochastic fracture model incorporating microstructural parameters

The present study proposed a model to predict Charpy impact absorbed energy for steel weld HAZ of predominantly upper bainite containing martensite-austenite constituent (MA) as a second phase. Probability distribution of local fracture stress for cleavage crack nucleation is calculated from microstructural parameters. On the other hand, local stress and strain are calculated from dynamic elastic-plastic finite-element analysis. Applying the weakest-link mechanism, probability of cleavage fracture is calculated. Charpy impact tests were conducted for steel simulated HAZ with various cooling rates. As a result, the predicted Charpy absorbed energy transition curves agreed well with the experiment and the model was validated.

Chemistry and three-dimensional morphology of martensite-austenite constituent in the bainite structure of low-carbon low-alloy steels

The martensite-austenite constituent (MA) originates from carbon-enriched and untransformed austenite surrounded by bainite and is known to deteriorate the toughness of bainitic steel. The relationship between MA and bainite formation in low-carbon low-alloy steel was investigated with respect to the effects of cooling rate and Nb addition, as well as the relationship between the MA distribution and surrounding bainite. Nb addition enhanced the formation of MA over a wider range of cooling rates, and the fraction of MA became maximum at a cooling rate of 5 K/s. A microstructure consisting of carbide-free coarse bainite with few high angle boundaries is formed near the prior austenite grain boundary, and fine bainite with a high density of carbide and high angle boundaries is formed within the austenite grains. MA tends to be located in the coarse bainite region. Three-dimensional observations of the bainite and MA indicated MA is elongated along bainitic ferrites with the same growth direction or the same habit plane, while MA is blocky when the surrounding bainitic ferrites have different growth directions. Thus, the shape of MA is strongly affected by the crystallography of the surrounding bainite structure. The carbon content in austenite revealed that the carbon content of MA is close to the T0 composition at the transformation temperature, which suggests that MA formation is caused by a reduction of the driving force for transformation by carbon enrichment, which results in incomplete bainite transformation. Nb addition further inhibits the decomposition of austenite and promotes MA formation.

Effect of morphologies of martensite–austenite constituents on impact toughness in intercritically reheated coarse-grained heat-affected zone of HSLA steel

By using the gleeble-3500 simulator, three different two-pass weld thermal cycles were applied to base metal of a high strength low alloy (HSLA) steel with the purpose of obtaining martensite-austenite (MA) constituents of different morphologies in intercritically reheated coarse-grained heat-affected zones (ICGHAZ). Morphology of each MA constituent was characterized by maximum length, maximum width and aspect ratio (maximum length/ maximum width). Toughness of thermal simulated specimens was examined by using an instrumented Charpy impact tester. Behaviour of cracks was present by Charpy impact curve and typical data. Correlation between behaviour of cracks and morphologies of MA constituents was further analysed by observing crack propagation and microstructure of MA constituent. Fracture modes of MA constituents with slender and massive shape were proposed. Results show that slender MA constituents are more harmful to toughness compared with massive ones.

The role of Cu and Al addition on the microstructure and fracture characteristics in the simulated coarse-grained heat-affected zone of high-strength low-alloy steels with superior toughness

ABSTRACTThe effects of Cu and Al addition on the microstructure and fracture in the coarse-grained heat-affected zone (CGHAZ) of high-strength low-alloy steels with superior toughness were studied and compared with the X70 pipeline base steel counterpart. The microstructure in base steel was dominated by a small fraction of acicular ferrite and predominantly bainite. However, acicular ferrite microstructure was obtained in Cu-bearing steel, which nucleated on complex oxide with outer layer of MnS and CuS because of Cu addition. The microstructure in Al-bearing steel consisted of bainite with ultrafine martensite–austenite constituent, which was refined by Al addition. CGHAZ in Cu-bearing and Al-bearing steels had superior impact toughness and ductile fracture, which were attributed to acicular ferrite and ultrafine martensite–austenite constituent, respectively.

In situ observation and electron backscattered diffraction analysis of granular bainite in simulated heat-affected zone of high-strength low-alloy steel

ABSTRACTIn this study, the formation of granular bainite (GB) which may form in the heat-affected zone of double-sided double arc welding joint was in situ observed. The crystallographic characteristics of GB were also compared with those of microstructure mainly comprising lath bainite (LB). The results show that bainite packets exist at the initial stage of GB transformation and can be distinguished by the distribution of retained austenite and martensite–austenite constituents. The bainite blocks of GB are larger than those of LB which makes both the length and fraction of high-angle grain boundaries in GB less than those of LB which partly lead to the brittleness of granular bainite.

Elemental distribution in the martensite–austenite constituent in intercritically reheated coarse-grained heat-affected zone of a high-strength pipeline steel

We have studied here the distribution of carbon and alloying elements in the martensite-austenite (M-A) constituent in intercritically reheated coarse-grained heat-affected zone (ICCGHAZ) of a high-strength pipeline steel using atom probe tomography (APT). Notable enrichment of C (0.49wt%) and Mn (2.32wt%) was observed within the M-A constituent, which induced the formation of lath martensite and deteriorated the toughness. Elemental segregation in the interfacial region between M-A constituent and matrix may contribute to the debonding mechanism of M-A constituent and assist nucleation of cleavage cracks. Distribution of solute Nb indicated no apparent difference between the matrix and M-A constituent.

The evolutions of microstructure and mechanical properties of 2.25Cr-1Mo-0.25V steel with different initial microstructures during tempering

The evolutions of microstructure and mechanical properties during tempering at 700°C, of normalized and oil-quenched 2.25Cr-1Mo-0.25V steel samples to simulate the central and surface parts of the industrial heavy wall forgings, respectively, have been investigated. It is found that the normalized sample has a granular bainite microstructure and the oil-quenched sample has a lath bainite microstructure. After 0.5h of tempering, the normalized sample has a higher strength and ductile-to-brittle transition temperature (DBTT) than the oil-quenched sample because of the strengthening effect of the undecomposed martensite-austenite (M-A) constituents and the presence of coherent tiny VC type precipitates in granular bainite. However, when the tempering time is increased from 0.5 to 128h, the strength as well as the DBTT of the normalized sample decreases more pronounced than that of the oil-quenched sample. This is attributed to the synergistic effect of the decomposition of M-A constituents, growth of VC type precipitate in the normalized sample, and the increase in the effective grain size in the oil-quenched sample.

Effects of Ni and Mn addition on critical crack tip opening displacement (CTOD) of weld-simulated heat-affected zones of three high-strength low-alloy (HSLA) steels

In order to understand and improve fracture toughness of heat affected zones (HAZs) of high-strength low alloy (HSLA) steels, complex microstructures including quasi-polygonal ferrite (QPF), acicular ferrite (AF), granular bainite (GB), bainitic ferrite (BF), and martensite-austenite (MA) constituent should be identified, quantified, and then correlated with critical crack tip opening displacement (CTOD). In this study, microscopic analysis methods were achieved for identification and quantitation of microstructures in the HAZs of three HSLA steels. The coarse-grained HAZ (CGHAZ) consisted of AF, GB, and BF together with a small amount of MA, while the inter-critically heated HAZ (ICHAZ) consisted of QPF, GB, and MA. In the CGHAZ, Ni promoted the formation of AF, while it prevented the formation of GB, and the addition of Ni resulted in very high critical CTOD. In the CGHAZ, both Ni and Mn promoted the formation of AF and prevented the formation of GB, while Ni was more effective than Mn. Thus, the addition of Ni resulted in very high critical CTOD. In the ICHAZ, both Ni and Mn promoted the formation MA. However, in the high-Ni-containing steel, a number of MAs were densified along Ni-segregated bands, and thus readily provided void initiation sites. This played an important role in reducing the mean free path for coalescence of voids and crack propagation, which easily led to the serious deterioration of critical CTOD.